There is a long felt but unmet need for compact, simple, economical, and effective means for separating pollutants from water, including salt from seawater.
Agriculture over large areas of the United States is impossible because of the brackishness of available water. Water quality also limits siting of facilities for electrical power production.
Industrial expansion is impossible without a large supply of soft water. For example, the production of a ton of aluminum requires over 300,000 gallons of water, and the production of a ton of artificial fibres requires 200,000 gallons. In food processing also, large amounts of soft water are required--one gallon of beer takes 350 gallons of water.
In addition to the need for pure feedwater, there is an even greater need for wastewater technology due to severe penalties imposed by federal and state law on producers of hazardous materials.
Purification technology for water and other liquids falls into four main categories: chemical separation, mechanical separation, reverse osmosis, and distillation.
Chemical separation includes the ion exchange process in which cations or anions are absorbed from an electrolyte solution and other ions are given off to the solution. Flocculation and precipitation of contaminants result from the addition of appropriate chemicals. The cost of chemicals, however, limits the use of chemical separation.
Mechanical separation devices include filters, flotation cells, centrifuges, and hydrocyclones. Such devices do not utilize a change of state to effect fluid separation. Present filtration technology for fine pollutants (ultrafiltration) relies on membranes, which tend to clog. Cross-flow filtration technology partially solves this problem by creating a flow across the filter surface and periodically backflushing to clear the membrane. Ultrafiltration technology is not able to achieve desalination.
Centrifuges and hydrocyclones are unable to remove viruses, bacteria, colloids, sugars, metal ions, pyrogens and other troublesome submicron contaminants because the specific gravity differential is insufficient.
Reverse osmosis technology is based on the phenomenon that pressure upon salt water will force fresh water to flow across a membrane. Desalination plants using reverse osmosis are now common. Reverse osmosis units are the most energy efficient by far of all other processes, requiring an expenditure of 6.6 KWh per cubic meter. However, fouling of the reverse osmosis membranes by oil or bacteria in the feedwater requires replacement of expensive components, so extensive pretreatment of the feedwater is necessary. The energy efficiency of reverse osmosis is offset by the high cost of installation and maintenance.
Distillation involves evaporating a distilland and then condensing a distillate from the vapor produced. Under prior art, even with vacuum distillation, heat is added to the distilland in order to achieve vapor pressure of the desired distillate.
But the heat that evaporates the distilland also facilitates undesirable chemical reactions in the distilland and scale formation on distiller surfaces. Scale necessitates chemical pretreatment of the distilland as well as frequent shutdowns for cleaning, which is probably why desalineation devices having intricate internal heat exchange surfaces have not found a wide application. Moreover, heat wastes an inordinate amount of energy.
Vacuum distillation reduces the heat needed to achieve a change of state in the distilland, but the lowered pressure causes cavitation within the distilland. The bursting of bubbles at the distilland surface sprays mist droplets, which are large enough to transport pollutants, into the vapor. The vapor must be scrubbed before condensation. The problem of entrained mist from vacuum distillation is presently addressed by towers and de-mister screens, which have the disadvantage of large size and the need for cleaning and replacement.
All distillation devices known to prior art require a distilland chamber for containing the distilland. Scaling, mist entrainment, energy efficiency, and undesirable chemical reactions due to heat are problems remaining to be solved in the arts of distillation and desalination.